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Computation of the Internet Checksum via Incremental Update (RFC1624)

IP.com Disclosure Number: IPCOM000002459D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2019-Feb-12
Document File: 6 page(s) / 7K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A. Rijsinghani: AUTHOR

Related Documents

10.17487/RFC1624: DOI

Abstract

This memo describes an updated technique for incremental computation of the standard Internet checksum. It updates the method described in RFC 1141. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 36% of the total text.

Network Working Group A. Rijsinghani, Editor Request for Comments: 1624 Digital Equipment Corporation Updates: 1141 May 1994 Category: Informational

Computation of the Internet Checksum via Incremental Update

Status of this Memo

This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Abstract

This memo describes an updated technique for incremental computation of the standard Internet checksum. It updates the method described in RFC 1141.

Table of Contents

1. Introduction .......................................... 1 2. Notation and Equations ................................ 2 3. Discussion ............................................ 2 4. Examples .............................................. 3 5. Checksum verification by end systems .................. 4 6. Historical Note ....................................... 4 7. Acknowledgments ....................................... 5 8. Security Considerations ............................... 5 9. Conclusions ........................................... 5 10. Author’s Address ..................................... 5 11. References ........................................... 6

1. Introduction

Incremental checksum update is useful in speeding up several types of operations routinely performed on IP packets, such as TTL update, IP fragmentation, and source route update.

RFC 1071, on pages 4 and 5, describes a procedure to incrementally update the standard Internet checksum. The relevant discussion, though comprehensive, was not complete. Therefore, RFC 1141 was published to replace this description on Incremental Update. In particular, RFC 1141 provides a more detailed exposure to the procedure described in RFC 1071. However, it computes a result for certain cases that differs

Rijsinghani [Page 1]

RFC 1624 Incremental Internet Checksum May 1994

from the one obtained from scratch (one’s complement of one’s complement sum of the original fields).

For the sake of completeness, this memo briefly highlights key points from RFCs 1071 and 1141. Based on these discussions, an updated procedure to incrementally compute the standard Internet checksum is developed and presented.

2. Notation and Equations

Given the following notation:

HC - old checksum in header C - one’s complement sum of old header HC’ - new checksum in header C’ - one’s complement sum of new header m - old value of a 16-bit field m’ - new value of a 16-bit field

RFC 1071 states that C’ is:

C’ = C + (-m) + m’ -- [Eqn. 1] = C + (m’ - m)

As RFC 1141 points out, the equation above is not useful for direct use in incremental updates since C and C’ do not refer to the actual checksum stored in the header. In addition, it is pointed out that RFC 1071 did not specify that all arithmetic must be performed using one’s complement arithmetic.

Finally, complementing the above equation to get the actual checksum, RFC 1141 presents the following:

HC’ = ˜(C + (-m...

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